White Yunzi resembles white jade
(ф 21.6mm, thickness 7.6mm-9.2mm),
black Yunzi are non-glossy black by itself (can be oiled), but if held against the light, they will appear dark green (black ф 21.9mm) and show a nice translucency:
Now here comes the test: If holding against the light and you see voids, resp. gas bubbles in it, you may have glass stones. (That's what I was told later)
Traditional Yunzi Go stones are not lense shaped (≠ Japanese stones), but half-flat.
Called "One side convex" on
http://go-gamestore.com/goequip/stones/stoneinfo.htm#Marble , where the say it can be mass-produced.
Chinese players like this for analyzing variations by turning the stones in question upside down. (e.g. cf. with the picture on Weiqi Tiandi, where 杨莲 Yang Lian seems to have turned around all stones)
Sold at e.g. http://www.yun-zi.com
mdh: I have a set of these and they are very nice. There is only one slight problem. They do no fit on a regular Goban very well. They are slightly larger then Japanese (Korean) stones and when the Go board gets crowded, They will overlap some. I have a couple boards, but I plan to make a homemade board just for these stones.
rubilia: If you by "regular board" refer to a Japanese board, the mismatching is no surprise, since Chinese boards are slightly bigger. I think, Korean boards (and stones) are again a little smaller than Japanese ones, but I am not sure about that.